Advances in plasma processing have facilitated growth in the semiconductor industry. During plasma processing, plasma may be generated to process a substrate. However, plasma has a tendency to expand beyond the wafer region. Thus, the inability to confine the plasma may result in uncontrollable substrate processing, which may result in substandard devices and/or defective devices.
To facilitate discussion, FIG. 1A shows a schematic of a plasma processing system 100. Plasma processing system 100 may be a single, double (DFC) or triple frequency RF capacitively discharge system. In an example, radio frequencies (RF) may include, but are not limited to, 2, 27 and 60 MHz. Plasma processing system 100 may be configured to include an upper electrode 102, which is generally grounded and has a voltage potential of zero. Also, plasma processing system 100 may include an electrostatic chuck 104, which may act as a lower electrode.
Consider the situation wherein, for example, a substrate 120 is being processed. During plasma processing, an RF power 116 may be applied to electrostatic chuck 104. RF power 116 may interact with a gas 118 to ignite a plasma 106 between electrostatic chuck 104 and grounded upper electrode 102. Plasma 106 may be employed to etch and/or deposit materials onto substrate 120 to create electronic devices.
Plasma 106 tends to expand beyond the wafer region (e.g., outside of the region between upper electrode 102 and electrostatic chuck 104). If plasma expands beyond the wafer region, plasma density may decrease and plasma processing may shift outside of the wafer region resulting in uncontrollable substrate processing. Since plasma 106 is best controlled within the wafer region, manufacturers have attempted to confine the plasma. In an example, Lam Research Corporation has attempted to perform plasma confinement by mechanically confining the plasma by employing confinement rings.
Plasma processing system 100 shows a plurality of confinement rings (112a, 112b, 112c, 112d, and 112e), which may be a set of parallel rings inside the processing chamber to prevent the plasma from forming in the outer region. As discussed herein, outer region refers to the area between the confinement rings and a reactor wall. Typically, confinement rings may be constructed of dielectric material such as quartz.
However, confinement rings may not be sufficient to confine plasma if the electric field is high enough to interact with gas 118 to ignite a plasma in the outer region. In an example, plasma 106 may have a voltage potential (Vp), which may be a self-induced potential of the plasma relative to ground. An electric field may be induced due to the differences between the voltage potential of plasma 106 (e.g., at plasma edge 108) and the voltage potential of a reactor wall 114, which is typically grounded and has a voltage potential of zero. Thus, if the difference is high enough, a strong electric field 110 may be created resulting in plasma being ignited in the outer region.
The electric field amplitude induced due to the voltage difference between voltage potential of plasma edge 108 and the voltage potential at reactor wall 114 may be expressed by Equation 1:
                              E          ≈                                    Vp              -              Vw                        d                          =                  Vp          d                                    [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, the electric field amplitude (E) may be proportional to the potential difference between voltage potential (Vp) at plasma edge 108 and voltage potential (Vw) at reactor wall 114. Given that reactor wall may be grounded, the voltage potential (Vw) at reactor wall 114 may be equaled to zero. Thus, the electric field amplitude (E) may be equaled to the voltage potential (Vp) at plasma edge 108 divided by the distance (d) between plasma edge 108 and reactor wall 114.
As shown in Equation 1, the electric field (E) may be higher if the voltage potential (Vp) at plasma edge 1108 increase or the distance (d) (i.e., the distance between the plasma edge and the reactor wall) decrease. The increase in electric field may lead to plasma striking in the outer region causing unintended plasma unconfinement.
To be competitive, manufacturers have attempted to increase plasma processing efficiency. In an example, manufacturers may want to increase efficiency in the etching process by increasing the etch rate. To increase the etch rate, higher RF power may be employed to increase the plasma density. However, as can be seen from Equation 1, higher RF power may result in a higher plasma potential (i.e., voltage potential at the plasma edge), which may result in a stronger electric field being generated. Thus, an increased in the RF power may result in an electric field with sufficient amplitude to ignite plasma in the confinement rings region. In another example, manufacturers may want to increase process efficiency and control by employing a higher gas flow rate at a given RF power to increase the etch rate. However, a higher gas flow rate may also increase the gas pressure and thus the likelihood of plasma striking in the outer region. Due to the electric field that may be generated in the outer region, substrate processing may be limited in term of the amount of RF power and/or gas flow rate that may be employed.
In addition, manufacturers may maintain their competitiveness by processing a target substrate in order to create more devices per processing cycle. However, increasing the substrate size may decrease the distance (d) between plasma edge 108 and reactor wall 114. As can be seen from Equation 1, the reduction in the distance (d) may also give rise to an increase in the electric field (E), thereby, increasing the possibility of plasma ignition in the outer region.
FIG. 1B shows an equivalent circuit model of FIG. 1A. During plasma processing, RF power 116 may be applied to electrostatic chuck 104. Since upper electrode 102 is grounded, a large part of the RF current may return to ground through upper electrode 102. At the edge of the plasma, RF current may return to ground via three different paths. The RF current may return to ground (e.g., reactor walls) by capacitively coupling with an upper electrode extension 182 and a lower electrode extension 180, which is shown as capacitors 21 and 22, respectively. The remaining RF current may return to ground by flowing through the confinement rings, as shown by capacitor 23.
Some manufacturers have attempted to reduce the electric field by reducing the capacitance formed by capacitor 23. Capacitance may be reduced by increasing the distance (d) between plasma edge 108 and reactor wall 114. However, the increase in the distance (d) may require an increase in the reactor size. Increasing the reactor size may also require modifying other tool components. The cost associated with changing the reactor and its component may be costly.
With the prior art plasma confinement arrangement, confinement rings may be able to prevent the plasma from expanding into the outer region. However, as manufacturers attempt to be competitive by increasing process efficiency and control (e.g., increase RF power, increase gas flow rate, and increase substrate size), the confinement rings may no longer be an effective plasma confinement tool as the electric field increases, thereby increasing the potential of plasma being ignited in the outer region. Also, the prior art solution of increasing the reactor size to reduce the electric field may be an expensive alternative and does not provide a solution for current plasma processing system owners.